The other day I was biking around and noticed a photo laying face-down in the street. Hours later when I returned to the spot, the photo was still there. Picking it up, I discovered it was a nude self-portrait of an unknown woman. Who was she? How’d it get in the street? Who was it taken for? Should I keep it? My find would be perfect for the magazine Found, Davy Rothbart’s cult sensation that honors weird things picked up in the street. His talk and slideshow comes to Contemporary Arts Forum this Thursday.

“That’s awesome,” says Mr. Rothbart when he hears of my find. “It really is amazing how many people these days are taking photos of themselves naked, and also how many people lose them.”

“Each item is a fragment of a story,” he says. “You can find page six of a letter and wonder what the first five were. It’s up to you to fill in the blanks. And the other part is how it got to the street. We’ve found torn up pieces of a love letter. Did the person who received it rip it up? Or did the person who wrote it rip it up before they sent it?”

These are the teased-out mystery narratives at the center of Found, which has been honoring the detritus of our society since 2000.

The origin story makes for a good yarn. It started in Chicago, where Mr. Rothbart was interning at Chicago Public Media and its show “This American Life.” Mr. Rothbart found a note on his car meant for somebody else, a “Mario” who was suspected of cheating. The note, from “Amber” concluded with “I hate you!” and then “P.S. Page me later.”

The passive-aggressive, unintentional comedy made its way to Mr. Rothbart’s refrigerator door, and soon others were chipping in with their collections of found weirdness.

Mr. Rothbart put together his favorites and published the first issue of Found that year. From a tiny pre-blog-Internet zine in Chicago, the idea really took off after the New Yorker wrote a story about it. Suddenly, a few submissions every month turned into a blizzard of mail.

“We get about 10 to 20 things sent to us every day,” says Mr. Rothbart. “They come from around the world. Some are hilarious, some are heartbreaking. They’re all endlessly fascinating.”

An average issue of Found contains notes, apologies, reminders, drawings, schoolwork (often bad), and photos, loads of photos. (The nude and rude ones found their way into a special “Dirty Found” issue.)

With the amount that Found publishes in its magazine and on its blog at foundmagazine.com it may not be surprising to find a few original owners have contacted Mr. Rothbart.

“More often people are mystified, like, ‘Why are people interested in my love life?’ ” Mr. Rothbart says. He did get an e-mail from a girl whose dating of two different guys at the same time resulted in the advice letter Mr. Rothbart found and printed in the first issue. She was still with one of the two guys years later, but the other was coming back in town again. “It made me think we should do a ‘Where Are They Now?’ issue,” he says.

On Thursday night, Mr. Rothbart will read a selection of his favorite letters, and invites the public to bring in their own found treasures.

“Keep your eyes to the ground,” advises Mr. Rothbart to the aspiring “finder” as they are called. “Nine times out of ten it won’t be interesting. But one will be a gem. Pick one up every week and you’ll be on to something.”